The present invention pertains to the art of x-ray diagnostic imaging. It finds particular application in conjunction with generating medical diagnostic images through regions of a patient that contain clip, i.e. a metallic or high density implant, such as a tooth filling, a metallic pin or plate, or other prosthetic devices. It is to be appreciated, however, that the invention will also find application in conjunction with the generation of images through animate and inanimate subjects which contain widely disparate CT numbers or radiant energy absorption cross-sections.
Heretofore, CT images through slices which included clips or portions with very high CT numbers, e.g. greater than 1700 (where -1000 represents air and 0 represents water), have had severe artifacts. More specifically, metal prosthetic devices, dental fillings, and other foreign objects with high CT numbers cause dark and white streaks to emanate from the area of the high density object in the reconstructed image. These streaks white out or black out anatomical structure along the flared paths of the streaks for a distance that may be several times the size of the high density object. The streaks obliterate and obscure the anatomical structure in the regions covered by the streaks.
Various attempts have been made to eliminate the streaks and display the previously obliterated soft tissue. In "An Algorithm for the Reduction of Metal Clip Artifacts in CT Reconstructions" Med. Phys. 8(6), Nov/Dec 1981, pp. 799-807, authors Glover and Pelc describe a "rub out" algorithm which eliminates the streaks and the metal clip and replaces the clip with fictitious soft tissue. For each view, Messrs. Glover and Pelc process rays through the clip based on the nearly average views. In the average view, they match the data value along rays that do not intersect the clip to a polynomial to generate a baseline. Each baseline is subtracted from the average to define a signature. The signature is subtracted from the corresponding original view to create a corrected view. When the corrected views are reconstructed by filtered backprojection, an image is generated with the metal clip gone, completely replaced with soft tissue. The complete loss of the metal clip in this technique has significant drawbacks. For example, the exact position and presence of the metal clip is important information in various surgical and other corrective techniques taken as a result of the CT examination. More importantly, the streaking is reduced but not removed.
In "Image Reconstruction from Limited Data" presented at the International Workshop on Physics and Engineering in Medical Imaging, Mar. 15-18, 1982, Messrs. Medoff, Brody, and Macovski proposed an iterative correction. This article suggests comparing each pixel of an image reconstructed from the original data with a constraint, e.g. intensity. The intensities are adjusted in accordance with a priori information. This adjusted image is forward projected to generate a corrected set of line integral data. This article suggests that line integrals representing rays which do not intersect the metal clip are considered to be good and are not changed. The line integrals corresponding to the rays which do intersect the clip are removed and replaced by the corresponding line integral of the adjusted image. The data is reconstructed into an image which is inspected, and the correction process repeated iteratively.
In "Reduction of CT Artifacts Caused by Metallic Implants" Radiology, 164(2) pp. 576-577, August 1987, authors Kalender, Hebel, and Ebersberger suggested a linear interpolation technique. They noted that in the line integral corresponding to each ray through the clip, the clip was represented as a high, relatively narrow spike. In each line integral corresponding to a ray that intersected the clip, the authors deleted the spike and replaced it with a linear interpolation of the data to either side of the spike. This technique still results in noticeable artifacts.
The applicants herein believe that these artifacts in the prior art techniques are due at least in part to the polychromatic nature of the x-ray beam, commonly denoted as beam hardening. The CT projection or line integral data is commonly corrected for beam hardening such that soft tissue data can be treated as if it were generated by a substantially monochromatic beam. However, metal, and to some extent bone, are sufficiently more dense that the beam hardness correction is inaccurate. Replacing the metal clip portion of the ray with the linear interpolation of the adjoining tissue incorporates these beam hardening errors into the correction resulting in artifacts.
The present invention contemplates a new and improved technique for reducing clip artifacts.